The tech world is full of mysteries. We try to answer one of the least …

We hear the stories every few months about some poor sap who thinks he's buying a new gadget from the store, only to discover a next-gen set of cement chips once he gets home and peels the plastic off. But how (and why) does this kind of thing happen? We spoke to three former retail employees to find out.

Whodunnit

The thing we learned right off the bat is that this phenomenon has been going on for many, many years, though it received more coverage lately because of the blogosphere's popularity explosion. With more people buying music players, laptops, and other gadgets than ever and with more venues available to vent about bad experiences online, it's easy to raise a public outcry when this kind of thing happens.

"We had it happen a few times (mainly replaced by old or broken merchandise), and it was usually a judgment call regarding whether it was credible or not," said one former Cambridge Soundworks employee.

The general consensus, however, is that customers themselves are responsible for the large majority of these cases. People purchase an expensive item, take it home, replace it with bricks, and sometimes even shrinkwrap the box for a return. Many retail stores won't check a box that looks like it was never opened in the first place, making this an easy switch to pull.

"If you get the right customer service rep who's had a bad day—or perhaps even someone you know at the store—then return an item at the right time, they don't check the box and you get your money back," a former Circuit City employee named David told Ars. "Maybe the store is really busy and everyone is in a rush, and people just assume that with many electronics, the weight is enough proof."

Our three-man panel was split on the likelihood of employees being responsible for the switcheroo. David said that his time at Circuit City exposed him to lax return policies from manufacturer warehouses, and he said that there were many times when the store itself could just shrinkwrap a returned item to sell with an open box discount.

"You learn a few other tricks and you can have your own little electronics trade going on," he said. But the Cambridge Soundworks employee disagreed: "Employees have much easier ways of stealing merchandise—there's enough material there for another article altogether!"

Regardless of who's responsible, a former Apple Store employee also named David felt strongly that there's a reason certain chains have this problem more than others—varying inventory management policies. Apple apparently experiences this problem so little (if at all) that it's not even on the radar. Every single returned item must be inspected by a Genius at an Apple Store before taking it back, and Circuit City David agreed that items that have been shrinkwrapped at home still display telltale signs of being opened—an observant employee should be able to identify tampering even with a box that is still wrapped. The key is the "observant" part.

As for employee theft, Apple keeps its stockroom locked with more locked cages inside, and employees must sign out a key from a manager before gaining access to product. And that's not all. "All employees' bags had to be searched by a manager prior to leaving the store at the end of a shift," Apple David said. "Any personal tech gear had to be registered with the store, and a manager would issue a 'personal technology card' with your items and their serial numbers, which would be checked every time you left the building with said gear."

Still, when you find yourself with a box full of masonry instead of a laptop, the store's lax inventory management policies are often at the bottom of your list of worries for the moment. Everyone we spoke to recommended either opening the box in the store ("You might look like a douche when stepping aside to open your new gadget, but making sure you actually got your money's worth is rarely a bad idea," Circuit City David said), or calling immediately upon discovery. One person also suggested that requesting another unit instead of a refund can go a long way to back up your claim that you're a victim and not the victimizer.

We have many readers who spent at least some time selling electronics in their youth—what are your experiences with this kind of thing? Can you top the meat story? Have any tips to offer cautious buyers?